The present invention relates to a lubricating oil composition for use in two-stroke cycle cylinder injection engines, to be fed directly to the suction system or lubricated parts. More specifically, the invention relates to a lubricating oil composition for use in two-stroke cycle cylinder injection engines, which is superior in piston cleanliness, ring sticking prevention, and lubricity and which yields less carbon deposition of the exhaust system.
For two-stroke cycle cylinder injection engines, for example, it is a practice to adopt a method in which lubricating oil is fed midway of the suction system so that a mixture of fuel and lubricating oil is fed to the engine, or a method in which lubricating oil is injected and fed directly to lubricated parts such as the piston sliding surface, crank journal, and the like. Whichever method is adopted, the lubricating oil will burn together with the mixed gas of fuel and air, where the resulting exhaust gas contains unburnt lubricating oil such that particulate substances composed mainly of the unburnt lubricating oil and the fuel can be viewed by the eye as exhaust smoke. To reduce this exhaust smoke, it is effective to reduce the ratio of lubricating oil to fuel (mixing ratio). Also, there have been proposed lubricating oils improved with the aim of reducing the exhaust smoke.
However, the more the mixing ratio is reduced, the more the bearings and other parts of the engine will be subject to discoloration, wear, and the like, resulting in poorer lubricity. This means that the method by reducing the mixing ratio has limitations. Also, as is the case with the two-stroke cycle cylinder injection engines, when much carbon (soot) is contained particularly in the combustion gas and besides the piston is burdened with a high thermal load, there often occur such problems as the ring sticking that carbon is deposited between piston ring and ring grooves, causing the ring to stick thereto.